The present invention relates generally to an improvement of an optical system for recording an image on a recording medium, and particularly to an optical system for use in a so-called flying-spot scanning system.
Flying spot scanning systems are well known, one of which is shown in the U.S. Pat. No. 4,080,634, where a deflection mirror is employed to direct a laser beam to and fro. In order to improve a scanning speed, it is generally known that a recording medium is scanned by a plurality of beams, which is taught e.g. in "Electronics" (published Oct. 7, 1985), pages 40-42.
In a conventional apparatus an f-theta lens or f-tangent theta lens is usually employed as an objective lens and is disposed between a deflection mirror and a recording medium. The f-theta lens has such characteristic that a distance between a scanning spot of a scanning beam and an optical axis of the objective lens is directly proportional to an angle formed by the beam entering the lens with respect to the optical axis (hereinafter referred to as "deflection angle"), while the f-tangent theta lens has such characteristic that a distance between a scanning spot of a scanning beam and an optical axis of the objective lens is directly proportional to the tangent of a deflection angle. Further, an arcsine-theta lens may also be employed in the case where the deflection mirror is driven by a galvanometer. The arcsine-theta lens has such characteristic that a scanning speed of a scanning spot deflected by a deflection mirror is kept constant.
When a single beam is used in such a manner as the beam deflected by a deflection mirror forms a plane in which the optical axis of the objective lens is included (hereinafter such plane is referred to as "deflection plane"), a scanning locus thereof will be made straight as shown in FIG. 1-(A). However, when a plurality of beams are used in combination with such an objective lens as an f-theta lens, f-tangent theta lens or arcsine-theta lens, scanning beams will cause such as scan-line bow as shown in FIG. 1-(B), since the beams entering the objective lens will form more or less an inclination relative to the deflection plane. Accordingly, it is necessary to make a fine adjustment of respective scanning beams, which is obviously time-consuming and hence inefficient in manufacturing an apparatus.
Furthermore, in the case where a plurality of scanning beams are employed in the optical system, only a single beam corresponding to the deflection plane can ensure a scanning locus thereof straight, and remaining beams will cause the scan-line bow, as clearly shown in FIG. 1-(B). Due to such scan-line bow, a recording medium cannot be scanned uniformly by a plurality of the scanning spots, and accordingly it is difficult to apply such multiple scanning to a practical use.